Decoding a combined amplitude modulated and frequency modulated signal

ABSTRACT

The present disclosure relates to a method for decoding a combined AM/FM encoded signal, comprising the steps of: combining said encoded optical signal with light from a local oscillator configured with a local oscillator frequency; converting the combined local oscillator and encoded optical signal into one or more electrical signals by means of at least one opto-electrical converter having a predefined frequency bandwidth, thereby providing an amplified and encoded electrical signal having one or more encoded signal current(s), where one type of states have a higher oscillation frequency than other type of states; rectifying the encoded signal current(s), thereby obtaining an encoded power spectrum, wherein said power spectrum has different states, such as “0”-states and “1”-states, with different power levels such that they can be discriminated, said local oscillator frequency is defined by a positive local oscillator frequency-offset from the frequency of one of the states in said encoded optical signal, and said local oscillator frequency-offset is selected to be dependent on said frequency bandwidth.

FIELD OF INVENTION

The present disclosure relates to encoding and decoding a combinedamplitude modulated (AM) and frequency modulated (FM) signal.

BACKGROUND OF INVENTION

A communication system is the generation, transmission, reception anddecoding of information, which may be represented as a series of“0”-states and “1”-states; and is extremely important for society. Inoptical communication systems using directly modulated lasers, frequencychirping may occur. This is an effect causing the optical wavelength (orfrequency) to be dependent on the optical power. In other words, thewavelength (or frequency) of a “0”-state will differ from that of the“1”-state. Frequency chirp in communication systems are consideredundesirable and far from optimal, as it limits transmission distance dueto chromatic dispersion in the transmission fiber which converts thechirp-induced frequency broadening to time-broadening which causesneighboring symbols to overlap and therefore to be erroneously decoded.As a consequence, frequency chirped modulation is not preferred inoptical communication systems. Rather, modulators or lasers are madesuch that frequency chirping is suppressed enough to be used for opticalcommunication systems.

Chirp-free operation requires either chirp-managed lasers or externalmodulators; both of which come with high price and increased powerconsumption and heat. On the other hand, modulators or lasers withfrequency chirping come with low cost. There is therefore an economicalasset if frequency chirped modulators can be tolerated better in forexample optical communication systems.

There are solutions, which allow frequency chirped modulators to be usedin optical communications systems, but these have means for compensatingthe frequency chirping. This means that for example filters or morecomplex hardware solutions are required to be implemented in such a waythat the dynamic line width broadening is being eliminated and hence notexploited.

What is missing is a communication system that not only toleratesfrequency chirped modulation, but also takes advantage of the dynamicline width broadening inherent in the frequency chirped modulation.

In optical communication systems, there are two well-known detectiontechniques:

-   -   Direct detection    -   Coherent detection

Direct detection is the detection of amplitude only whereas coherentdetection is the detection of both amplitude and phase. Coherentdetection has many advantages over direct detection, including highersensitivity than direct detection and is therefore increasinglypreferred in long-reach (core network) communication systems wheretransceiver cost is shared by a high number of users opposed to metroand access networks which are very sensitive to transceiver cost.Coherent detection requires, however, information of the carrier phaseas the signal is demodulated by a local oscillator (LO) controlled by aphase-locked loop that serves as an absolute phase reference. Operationwith phase locked loops puts strict requirements on the system side. Twowell-known requirements for phase locked loops are:

-   -   Synchronisation between the LO and the encoder    -   Narrow optical line width of the LO and the encoder

If these requirements are not met, coherent detection is not workingproperly. The phase locked loop can be made either in the optical(analog) domain or in the digital domain with digital signal processing(DSP). Regardless of how the phase locked loop is implemented, coherentdetection is always required to operate with high cost lasers withnarrow optical line widths. In future optical communications systems formetro and access networks, there is a need for a detection techniquethat provides a low cost solution.

SUMMARY OF INVENTION

In order to address and solve the above described problems and needs,the present disclosure relates to a communication system using andexploiting frequency chirping. Specifically, the present disclosurerelates to a communication system that provides encoding and decoding ina communication system, where a phase-locked loop may be possible toeliminate, and thus provide a low cost solution for opticalcommunications systems.

The present disclosure describes signalling employing a combinedamplitude modulation (AM) and frequency modulation (FM), such asobtained with a frequency chirped laser, and decoding this combined AMand FM encoded signal, comprising the steps of: combining said encodedoptical signal with light from a local oscillator configured with alocal oscillator frequency; converting the combined local oscillator andencoded optical signal into one or more electrical signals by means ofat least one opto-electrical converter having a predefined frequencybandwidth, thereby providing an amplified and encoded electrical signalhaving one or more encoded signal current(s), where one type of stateshave a higher oscillation frequency than other type of states;rectifying the encoded signal current(s), thereby obtaining an encodedpower spectrum, wherein said power spectrum has different states, suchas “0”-states and “1”-states, with different power levels such that theycan be discriminated, said local oscillator frequency is defined by apositive local oscillator frequency-offset from the frequency of one ofthe states in said encoded optical signal, and said local oscillatorfrequency-offset is selected to be dependent on said frequencybandwidth.

The present disclosure describes signalling, comprising the steps ofencoding the optical signal by amplitude and frequency modulation, anddecoding the combined AM and FM signal, and wherein the encoding ordecoding of a combined AM and FM signal is using two or more levels. Twolevels, such as “0”-states and “1” states are typically used, such thatthe different states are separated in frequency and amplitude, but acommunication systems may also employ an alphabet comprising more thantwo states. This is typically denoted “advanced modulation format”,“higher-order modulation format” or “multilevel modulation format”. Theadvantage is that by using more than two states, it is possible toencode more than one bit of information into a single symbol. Asexamples, a system employing four amplitude levels will be able toencode two bits pr. symbol, a system employing four frequency levelswill be able to encode two bits per symbol, and a system whichindependently employs four amplitude and four frequency levels will beable to encode four bits pr. symbol. Apart from amplitude and frequency,the information may also be encoded in the phase of the carrier, in thepolarization of the carrier, as variations in pulse-width or asvariations in pulse position.

The present disclosure also provides a detector system for decoding acombined AM and FM encoded optical signal comprising at least twodifferent types of states, such as “0”-states and “1”-states,comprising: a local oscillator configured with a local oscillatorfrequency; a coupling device configured for coupling the encoded opticalsignal with light from the local oscillator; one or more opto-electricalconverter(s) having a predefined frequency bandwidth, configured forproviding an amplified and encoded electrical signal having one or moreencoded signal current(s) where one type of states have a higheroscillation frequency than another type of states; a rectifierconfigured for rectification of said signal current(s) to provide apower spectrum, wherein said power spectrum has different states, suchas “0”-states and “1”-states, with different power levels such that theycan be discriminated, said local oscillator frequency is defined by apositive local oscillator frequency-offset from the frequency of one ofthe states in said encoded optical signal, and said local oscillatorfrequency-offset is selected to be dependent on said frequencybandwidth.

Additionally, the detector may comprise a low pass filter configured forreducing the residual power of one type of states relatively to anothertype of state, such as “0”-states and “1”-states, with different powerlevels which can be discriminated more easily.

Accordingly, the present disclosure further relates to an opticalcommunication system comprising at least one transmitter and at leastone receiver comprising the herein disclosed detector system.

LIST OF FIGURES

FIG. 1 shows an embodiment of a spectrum of a combined AM and FM signalbefore light from the local oscillator is combined with the signal.

FIG. 2 shows an embodiment of a spectrum of a combined AM and FM signalbefore and after light from the local oscillator is combined with thesignal, also called beating.

FIG. 3 shows an embodiment of a signal level before rectification.

FIG. 4 shows an embodiment of a signal level after rectification and lowpass filtering.

FIG. 5 shows embodiments of half-wave and full-wave rectification of anRF signal.

FIG. 6 shows an embodiment of the present disclosure.

FIG. 7 shows an example of detuning of a local oscillator according tothe present invention using a photodiode with the same bandwidth as thebitrate of the system (back-to-back).

FIG. 8 shows an example of detuning of a local oscillator according tothe present invention using a photodiode with a bandwidth of 1.5 timesthe bitrate of the system (back-to-back).

FIG. 9 shows an example of detuning of a local oscillator according tothe present invention using a photodiode with same bandwidth as thebitrate of the system (100 km SSMF).

FIG. 10 shows an example of detuning of a local oscillator according tothe present invention using a photodiode with a bandwidth of 1.5 timesthe bitrate of the system (100 km SSMF).

FIG. 11 shows an example of how the receiver sensitivity at a BER of10e-9 depends on the LO detuning.

FIG. 12 shows an example of how the FM shift depends on the peak-to-peakvoltage of the data signal used to drive the VCSEL.

FIG. 13 shows an example of how the AM extinction ratio depends on thepeak-to-peak voltage of the data signal used to drive the VCSEL.

FIG. 14 shows an example of the optimum drive amplitude and resultant FMshift at 5 Gbps back-to-back and after 100 km SSMF transmission as afunction of PD bandwidth.

FIG. 15 shows an example of how the frequency of the zero-level (F0) andone-level (F1) depends on the drive amplitude.

FIG. 16 shows an example of how the optimum LO frequency-offset from thesignal center frequency and F1 varies with the drive amplitude for a 5Gbps and 7.5 Ghz photodiode (back-to-back).

FIG. 17 shows an example of how the optimum LO frequency-offset from thesignal center frequency and F1 varies with the drive amplitude for a 5Gbps and 7.5 GHz photodiode (100 km SSMF)

FIG. 18 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for aback-to-back system, 5 Gbps.

FIG. 19 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for aback-to-back system, 5 Gbps.

FIG. 20 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for a 40km SSMF system, 5 Gbps.

FIG. 21 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for a 40 kmSSMF system, 5 Gbps.

FIG. 22 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for a 80km SSMF system, 5 Gbps.

FIG. 23 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for a 80 kmSSMF system, 5 Gbps.

FIG. 24 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for aback-to-back system, 10 Gbps.

FIG. 25 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for aback-to-back system, 10 Gbps.

FIG. 26 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for a 40km SSMF system, 10 Gbps.

FIG. 27 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for a 40 kmSSMF system, 10 Gbps.

FIG. 28 shows an example of how the optimum FM shift depends on thephoto-detector bandwidth and transmission distance for 5 Gbps systems.

FIG. 29 shows an example of how the optimum FM shift depends on thephoto-detector bandwidth and transmission distance for 10 Gbps systems.

DETAILED DESCRIPTION OF THE INVENTION

Eliminated Phase Locked Loop

Since coherent detection per se requires a phase locked loop of a localoscillator, the present disclosure may be seen as different fromcoherent (synchronous) detection. The present disclosure may rather bedefined as asynchronous detection, meaning that the local oscillator maybe operating without a phase locked loop or may be operating withoutbeing synchronized to the signal. Whereas coherent detection systemsinclude some kind of phase locked loops, either analog or digital, thepresent disclosure does not require neither analog nor digital phaselocked loops. One advantage of the present disclosure is thus theability to eliminate the need of analog/digital phase locked loops. Inthe present disclosure, the phase-locked loop may be eliminated bytaking advantage of frequency chirping. The present disclosure uses asignal that is both amplitude and frequency modulated, such as obtainedwith a frequency chirped modulator, and such that this, in combinationwith the decoding of the combined AM and FM signal, may eliminate theneed for a phase locked loop. The FM is responsible for splitting thelevels into different frequencies, whereas the AM is responsible forsplitting the levels into different powers. Hence, combining AM and FMsignalling implies that the encoded signal is given additionalinformation about the different states encoded by the FM signal. It isthe combination of rectification and the combined AM and FM signal thatmay eliminate the need for phase locked loop. Whereas previous beliefstates that frequency chirped communication systems provide asub-optimal solution, the present disclosure exploits frequency chirpingand provides an optimal solution to frequency chirped communicationsystems with increased receiver sensitivity, wavelength (channel)selectivity, and improved transmission performance. In this way, thepresent disclosure relates to a communication system that provides theadvantages of coherent detection, i.e. increased receiver sensitivity,wavelength (channel) selectivity, and improved transmission performance.Furthermore, the present disclosure relates to a communication systemwithout the drawbacks of coherent detection; the present disclosureworks with frequency chirped modulation and does not require a phaselocked loop, neither analogue nor digital. As a consequence, it maytherefore be possible to use low cost lasers with broad line widths asthe local oscillator, but also as the AM and FM transmitter/encoder,thereby reducing the overall cost, in particular for future opticalcommunications systems. As an example, the present disclosure mayprovide a method and a system for asynchronous detection using avertical cavity surface emitting laser (VCSEL) as local oscillator (fordecoding) and a directly modulated VCSEL as transmitter or modulator(for encoding). Using a local oscillator operating without a phaselocked loop, may allow for a method and a system, where no need is for acomplex algorithm or hardware for phase-locked loops to be implemented.

In one embodiment, it is possible to use a local oscillator operatingwith a phase locked loop. Using it like this, the receiver becomes acoherent detector. Using a local oscillator operating with a phaselocked loop and the combined AM and FM transmission may still provide asystem and a method which yield improved chromatic dispersion toleranceand improved extinction ratio. It is therefore not a requirement thatthe local oscillator is operating without a phase locked loop, butrather an advantage.

Opto-Electrical Conversion

In one embodiment of the present invention, the step of converting theoptical signal into one or more electrical signals is providing by meansof at least one opto-electrical converter having a predefined frequencybandwidth.

Tolerance of Limited Amplitude Extinction Ratio

Using directly modulated devices may provide the ability to operate at alimited amplitude extinction ratio at the transmitter. As some of theresidual power in one state may be further reduced or removed by lowpass filtering, the present disclosure may be more tolerant to limitedamplitude extinction ratio.

Eliminated Dispersion Compensation

One advantage of the present disclosure may be the ability to operatewith the high dynamic frequency chirping of directly modulated lasers.Since the chirp-induced spectral broadening may be removed by theprocess of coupling with the LO, rectification and low-pass filtering,the achievable transmission distance using chirped lasers may beimproved. Another advantage of the present disclosure may be the abilityto reduce the chromatic dispersion effected spectrum. Thus the chromaticdispersion tolerance may be improved over direct detection methods.Another advantage of the present disclosure may be the ability toeliminate the need of analog/digital dispersion compensation.

Low Pass Filtering

The typical effect of a low pass filter is to remove ripples from asignal, and hence the role of the low pass filter may also be to cleanthe signal as in a conventional configuration. In some embodiments, itis the low pass filter in combination with the combined AM and FM signalthat allows for the ability to operate at a limited amplitude extinctionratio and also the ability to operate with the high dynamic frequencychirping of directly modulated lasers. Low pass filtering may be applieddigitally or analogically.

Threshold Detection

In one embodiment, it may be required to apply threshold to the powerspectrum such that different states, such as “0”-states and “1”-states,are automatically detected. In this way, it may be possible to getinformation about the encoded states. Threshold detection may beimplemented by using a threshold detection module, also called adecision circuit. Threshold detection may be applied digitally oranalogically.

Coupling

The coupling device may be a 3 dB coupler, a 6 dB coupler or a 90-degreehybrid or similar device. Various couplers or hybrids are possible touse, but the 3 dB coupler is in general simpler than a 90-degree hybrid,and hence a 3 dB coupler may be preferable. The one or moreoptical-electrical converters may be photo diodes.

Rectification

The rectifier is a device configured for performing rectification. Therectifier and the rectification is a part of the decoding. Rectificationmay be applied digitally or analogically. Using a rectifier may providereduced computational complexity and/or hardware and accordingly theoverall cost. For example, the rectifier may be used without ananalog/digital (A/D) converter. One advantage of the present disclosureis thus the ability to eliminate the need of analog/digital (A/D)converters. Rectification may be performed as half-wave rectificationsuch that either the positive or negative part of the signal is removed.Half-wave rectification may be possible with a gate with a non-lineartransfer function. A gate may be biased such that the e.g. negative partof the signal is lower than the threshold of the gate. Rectification mayalso be performed as full-wave rectification such as a squaring element,where all negative values are converted to positive values. Asdescribed, rectification may be squaring. This may be implemented inhardware or software. In the case, where implemented in software, ananalogue/digital converter may be implemented before processed in adigital signal processor (DSP). An alternative to squaring may beobtained by Hilbert transforming the signal. Various other solutions mayhowever be possible. Examples of analogue rectifiers include XOR gates,and diode bridges. Both the XOR gates and the diode bridges allow forreal-time signal processing without DSP, and may thus be preferred overa DSP.

Sensitivity

One advantage of the present disclosure may be that it provides aperformance improvement similar to that of coherent detection being ableto operate with 10-15 dB lower input than with direct detection. It isdue to the amplification from the local oscillator that this performancemay be achieved.

Encoding

In one embodiment of the present disclosure, the signal is encoded byone or more simultaneous AM and FM device, such as a frequency chirpedlaser and/or direct modulated laser, in particular a DML or a VCSEL.Hence, the transmitter is configured to generate a combined AM and FMsignal by one or more combined AM and FM device(s), such as a frequencychirped laser, in particular a DML or a VCSEL. Both DMLs and VCSELs havea broad linewidth and in general low cost.

In another embodiment of the present disclosure, the signal is encodedby one or more separate AM device(s) and one or more separate FMdevice(s) such that this allows for use of more advanced modulationformats with more amplitude and frequency levels. Hence, the transmitteris configured to generate a combined AM and FM signal by one or moreseparate AM devices and by one or more separate FM devices.

Regardless of how the combined AM and FM signal is generated, thefrequency modulation is responsible for the different states areconverted to different frequencies, whereas the amplitude modulation isresponsible for separating the different states in amplitude, therebyconveniently supplying further information of the different states asconventional systems do not include.

The different frequencies, i.e. the different states, are separated by afrequency separation, also called a FM shift. Thus, the FM shift isdefined as the frequency separation between the two states of thefrequency modulated (FM) signal. As an example, the FM shift is thedifference between the “0”-states and the “1”-state of the combinedAM-FM signal, i.e. the optical signal.

In one embodiment of the present invention, the frequency modulation isconfigured such that the frequency separation, i.e. the FM shift,between the states in the optical signal is less than 15 GHz, or lessthan 14 GHz, or less than 13 GHz, or less than 12 GHz, or less than 11GHz, or less than 10 GHz.

In another embodiment of the present invention, the frequency modulationis configured such that the frequency separation between the states inthe optical signal is dependent on the frequency bandwidth of theopto-electrical converter.

In yet another embodiment of the present invention, the frequencymodulation is configured such that the frequency separation between thestates in the optical signal is proportional with a proportionalityfactor to the frequency bandwidth of the opto-electrical converter.

In a preferred embodiment of the present invention, the proportionalityfactor is between 0.2 and 1.4, such as between 0.4 and 1.2, such asbetween 0.8 and 1.2, such as between 0.9 and 1.1, such as 1.

In some embodiments of the present invention, the proportionally factoris dependent on the transmission distance.

In other embodiments of the present invention, the proportionally factoris dependent on the transmission speed, defined by the data transferspeed, measured in Gbps.

The Signal

In one embodiment, the signal is an optical signal. In some embodiments,the signal may be an RF signal. Furthermore, the signal may be a signalin free space or in an optical fibre. Also, the signal may comprise oneor more wavelength channels.

In a most preferred embodiment of the present invention, the signal, forexample the optical signal is configured with an AM extinction ratiobetween 3 dB and 6 dB, preferably between 4 dB and 5 dB, more preferablyapproximately 4.5 dB. Using such a configuration may for example allowfor a simple setup of the transmission system.

The Local Oscillator

In one embodiment, the local oscillator is an uncooled laser, such as aDML and/or VCSEL. Whereas uncooled lasers are low cost, a high costtemperature controlled laser may also be used as the local oscillator.The local oscillator may be tuned to a frequency or a wavelength of thesignal. This can either be an in-band or an out-of-band configuration.In an in-band configuration, the LO is tuned to a frequency orwavelength within a spectrum of the signal. In an out-of-bandconfiguration, the LO is tuned to a frequency or wavelength outside aspectrum of the signal. In this way, wavelength selectivity may beachieved using the local oscillator. Using the local oscillator as awavelength selector implies that the present disclosure can be usedwithout filters. However, wavelength channels may be filtered by one ormore optical filters. By tuning the local oscillator to a frequency,where one type of state is located, the state may be up-converted to afrequency which may be lower than another up-converted state. The signalmay in general be up-converted to a frequency which is equal to theinstantaneous frequency difference between that of the signal and theLO. In some embodiments, the tuning may be system dependent; inparticular the tuning may be dependent on the temperature. Thus, atuning to a given state may include tuning the LO to frequency orwavelength inside or outside the spectrum.

The local oscillator may be used as wavelength selectivity means,similar to coherent detection, thereby eliminating the need of anoptical filter before the detector.

In one embodiment of the present invention, the local oscillator has afrequency higher than one of the states, where one of the states is astate with the highest amplitude.

In another embodiment of the present invention, the local oscillatorfrequency-offset is greater than the bandwidth of the opto-electricalconverter.

In yet another embodiment of the present invention, the local oscillatorfrequency-offset is selected to be between 1 and 1.5 times the bandwidthof the opto-electrical converter, most preferably approximately 1.2times the bandwidth of the opto-electrical converter. In the meaning ofapproximately, is here to be understood a deviation of up to 20%.

Error Detection

In another embodiment, error detection may advantageously be implementedfor system verification. Error detection may be implemented using anerror detection module, such as a bit-error-rate-tester.

Polarization Independence

In one embodiment of the present disclosure, it may be preferable toobtain polarization independence, for example if implemented incommercial systems. Several methods exist to obtain polarizationindependence. One method may be to use a polarization diversityreceiver, which may include splitting the signal and light from thelocal oscillator into two orthogonal polarizations, thereby obtainingfour channels, and then combining these four channels. Another way toobtain polarization independence may be to use polarization scrambling.Various other methods may be used. A third way to obtain polarizationindependence may be by adaptive polarization control, implying aligningthe polarization of the signal to that of the light. Alternatively,polarization independence may be obtained by aligning the polarizationof the light to that of the signal. In a preferred embodiment this maybe done automatically. This could for example be achieved by scanningand controlling the polarization of the LO. Alternatively, it may beachieved by scanning and automatically controlling the polarization ofthe signal, where the scanning and controlling may include amaximization of the combined signal. In a manual configuration, thepolarization of the signal or light may be polarized to that of thelight or signal using a manual polarization controller.

EXAMPLES Example 1—Spectrum Before Combination with LO

FIG. 1 shows an embodiment of a spectrum of a combined AM and FM signalbefore light from the local oscillator is combined with the signal. Thespectrum has two peaks corresponding to a “0”-state 0 and a “1”-state 1.The “0”-state 0 is separated from the “1” 1 state both in frequency andamplitude. The extinction ratio is the power ratio between the “0”-stateand the “1”-state.

Example 2—Spectrum after Combination with LO

FIG. 2 shows an embodiment of a spectrum of a combined AM and FM signalbefore and after light from the local oscillator is combined with thesignal, also called beating. It can be seen that the local oscillator istuned to a frequency, where the “1”-state 1 is located. The LO is tunedclose but not exactly to the “1”-state 1. The “1”-state is up-convertedto a frequency which is lower than the up-converted “0”-state 0. Theextinction ratio is the power ratio between the “0”-state 0 and the“1”-state 1. It is interesting to note that after beating the signalwith the light from the local oscillator, followed by rectifying thesignal, the “0”-state is lowered, thereby giving an improved extinctionratio. When the signal is in the “1”-state 1, the amplitude is high, andthe oscillation frequency is low. When the signal is in the “0”-state 0,the amplitude is low and the oscillation frequency is high.

Example 3—Signal Before Rectification

FIG. 3 shows an embodiment of a signal level before rectification. Thissignal is obtained using a 90 degree hybrid such that the signalcomprises in-phase and quadrature components. From this signal, thein-phase and quadrature components of the signal do not by themselvesgive information about the signal.

Example 4—Signal after Rectification

FIG. 4 shows an embodiment of a signal level after rectification and lowpass filtering. This signal is obtained using a 90 degree hybrid suchthat the signal comprises in-phase and quadrature components. Thein-phase and quadrature is combined to a single signal and thenrectified. From this signal, the rectified signal gives informationabout the signal. Information regarding “0”-state and “1”-state ismeaningful and can be determined using threshold detection.

Example 5—Rectification

FIG. 5 shows embodiments of half-wave and full-wave rectification of anRF signal. Using half-wave rectification implies that half of the signalis erased.

Example 6—A System

FIG. 6 shows an embodiment of the present disclosure. A combined AM/FMencoded signal 2 is together with light from a local oscillator 3,combined into a coupler 4 into two electro-optical converters 5, whichconvert the signal into two electrical signals and passes them furtherinto a rectifier 6, where the electrical signals are decoded.

Example 7—Detuning of Local Oscillator Using a Photodiode with the SameBandwidth as the Bitrate of the System (Back-to-Back)

FIG. 7 shows an example of how the bit error rate (BER) depends on thereceiver input power for various local oscillator frequencies. Theexample as shown is modelled. In this example, the relationship is shownfor a back-to-back. In this example, the bandwidth of the photodiode is5 GHz, which is equal to the bitrate of the system (5 Gbps). Optimumvalues of the LO frequency and drive amplitude are found. The driveamplitude is fixed at this optimum, and the LO frequency is variedaround its optimum value. Accordingly, the receiver sensitivity at a BERof 10e-9 can for example be found for a given detuning. This exampleshows that the opto-electrical converter, in this case the photodiode,has a predefined frequency bandwidth. Thus, the predefined frequencybandwidth for the photodiode is proportional to the bitrate of system,and in this example, the proportionality factor is 1.

Example 8—Detuning of Local Oscillator Using a Photodiode with aBandwidth of 1.5 Times the Bitrate of the System (Back-to-Back)

FIG. 8 shows an example of how the bit error rate (BER) depends on thereceiver input power for various local oscillator frequencies. Theexample as shown is modelled. In this example, the relationship is shownfor a back-to-back. In this example, the bandwidth of the photodiode is7.5 GHz, which is 1.5 times the bitrate of the system (5 Gbps). Optimumvalues of the LO frequency and drive amplitude are found. The driveamplitude is fixed at this optimum, and the LO frequency is variedaround its optimum value. Accordingly, the receiver sensitivity at a BERof 10e-9 can for example be found for a given detuning. This exampleshows that the opto-electrical converter, in this case the photodiode,has a predefined frequency bandwidth. Thus, the predefined frequencybandwidth for the photodiode is proportional to the bitrate of system,and in this example, the proportionality factor is 1.5.

Example 9—Detuning of Local Oscillator Using a Photodiode with the SameBandwidth as the Bitrate of the System (100 km SSMF)

FIG. 9 shows an example of how the bit error rate (BER) depends on thereceiver input power for various local oscillator frequencies. Theexample as shown is modelled. In this example, the relationship is shownfor a back-to-back. In this example, the bandwidth of the photodiode is5 GHz, which is equal to the bitrate of the system (5 Gbps). Optimumvalues of the LO frequency and drive amplitude are found. The driveamplitude is fixed at this optimum, and the LO frequency is variedaround its optimum value. Accordingly, the receiver sensitivity at a BERof 10e-9 can for example be found for a given detuning. This exampleshows that the opto-electrical converter, in this case the photodiode,has a predefined frequency bandwidth. Thus, the predefined frequencybandwidth for the photodiode is proportional to the bitrate of system,and in this example, the proportionality factor is 1.

Example 10—Detuning of Local Oscillator Using a Photodiode with aBandwidth of 1.5 Times the Bitrate of the System (100 km SSMF)

FIG. 10 shows an example of how the bit error rate (BER) depends on thereceiver input power for various local oscillator frequencies. Theexample as shown is modelled. In this example, the relationship is shownfor a back-to-back. In this example, the bandwidth of the photodiode is7.5 GHz, which is 1.5 times the bitrate of the system (5 Gbps). Optimumvalues of the LO frequency and drive amplitude are found. The driveamplitude is fixed at this optimum, and the LO frequency is variedaround its optimum value. Accordingly, the receiver sensitivity at a BERof 10e-9 can for example be found for a given detuning. This exampleshows that the opto-electrical converter, in this case the photodiode,has a predefined frequency bandwidth. Thus, the predefined frequencybandwidth for the photodiode is proportional to the bitrate of system,and in this example, the proportionality factor is 1.5.

Example 11—LO Detuning Penalty

FIG. 11 shows an example of how the receiver sensitivity at a BER of10e-9 depends on the LO detuning. The example as shown is modelled. Inthis example, the dependency is based on the data from FIG. 7-10. Thisexample shows that using a photodiode with a bandwidth of 1.5 times thebit rate of the system improves receiver sensitivity. This improvementis more pronounced in the low-dispersion (back-to-back) case. Thus, thisexample has shown that LO detuning is increased by increasing thephotodiode bandwidth. Further, dispersion shifts the LO detuning frombeing symmetric around the optimum LO frequency towards being moretolerant to positive than to negative detuning values. The 1-dBtolerances are listed in the table below the graph.

Example 12—Frequency Modulation (FM Shift) vs VCSEL Drive Amplitude

FIG. 12 shows an example of how the FM shift depends on the peak-to-peakvoltage of the data signal used to drive the VCSEL. The example as shownis modelled. As can be seen from the graph in FIG. 12, there is a linearrelationship between the FM shift and the drive amplitude of the VCSEL.It has been found that the relationship is independent on the bitrate.

Example 13—AM Extinction Ratio vs VCSEL Drive Amplitude

FIG. 13 shows an example of how the AM extinction ratio depends on thepeak-to-peak voltage of the data signal used to drive the VCSEL. Theexample as shown is modelled. As can be seen from the graph in FIG. 13,there is a linear relationship between the AM extinction ratio and thedrive amplitude of the VCSEL. It has been found that the relationship isindependent on the bitrate.

Example 14—Optimum FM Shift vs PD Bandwidth

FIG. 14 shows an example of the optimum drive amplitude and resultant FMshift at 5 Gbps back-to-back and after 100 km SSMF transmission as afunction of PD bandwidth. The example as shown is modelled. It can beseen that for low dispersion (back-to-back), the optimum FM shift isalmost equal to the bandwidth. For high dispersion (100 km SSMF), theoptimum FM shift is almost constant with increasing PD bandwidth. Theadvantage of a high FM shift is countered by an increasing dispersionpenalty due to the increased optical signal bandwidth for high FM shift.

Example 15—Optimum LO Frequency-Offset from 1-Level Frequency

FIG. 15 shown an example of how the frequency of the zero-level (F0) andone-level (F1) depends on the drive amplitude. The example as shown ismodelled. The frequency is normalized to the frequency of theunmodulated VCSEL, i.e. F0=F1=0. Both F0 and F1 depend linearly on thedrive amplitude. F1 moves towards higher frequencies, whereas F0 movesto lower frequencies. The center frequency in-between moves slightlytowards lower frequencies. This is due the adiabatic chirp caused by theheating of the VCSEL due to the RMS power of the VCSEL drive signal. Inone embodiment of the present invention, the center frequency shift iseliminated by temperature controlling VCSEL.

Example 16—Optimum LO Frequency-Offset from the Signal Center Frequencyand F1 (Back-to-Back)

FIG. 16 shows an example of how the optimum LO frequency-offset from thesignal center frequency and F1 varies with the drive amplitude for a 5Gbps and 7.5 Ghz photodiode (back-to-back). The example as shown ismodelled. It can be seen that the LO should be tuned to have a constantoffset from F1 irrespective of the drive amplitude and thereby alsoindependent on the FM shift.

Example 17—Optimum LO Frequency-Offset from the Signal Center Frequencyand F1 (100 km SSMF)

FIG. 17 shows an example of how the optimum LO frequency-offset from thesignal center frequency and F1 varies with the drive amplitude for a 5Gbps and 7.5 GHz photodiode (100 km SSMF). The example as shown ismodelled. It can be seen that the LO frequency is almost the same forall drive amplitudes and therefore for all values of the FM shift. Inother words, this example shows the opposite of the example given inexample 16. From this example, it can be seen that the optimum LOfrequency is dependent dispersion.

Example 18—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 1.5 Times the Bitrate (Back-to-Back, 5Gbps)

FIG. 18 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for aback-to-back system. The example as shown is modelled. Results fordifferent fixed AM extinction ratios are also included in the modelledresults. Thus, in this example, it is shown that the frequency of thelocal oscillator is selected to have a predefined offset from,preferably higher than, the frequency of one of the states in theencoded optical signal, preferably the state with the highest amplitude.It is further shown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 7.5 GHz, and the offset is between 7-9 GHz.Thus, in this example, the offset is selected to be between 0.9 and 1.2times the bandwidth of the opto-electrical converter. From this example,it can be seen that optimum LO frequency-offset varies little with theFM shift. In other words, the offset is varying little in the rangebetween approximately 1 and 1.5 times the bandwidth of theopto-electrical converter. In this example, it has been shown that forlow dispersion, the optimum LO frequency-offset is independent of the AMextinction ration.

Example 19—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 2 Times the Bitrate (Back-to-Back, 5Gbps)

FIG. 19 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for aback-to-back system. The example as shown is modelled. Results fordifferent fixed AM extinction ratios are also included in the modelledresults. Thus, in this example, it is shown that the frequency of thelocal oscillator is selected to have a predefined offset from,preferably higher than, the frequency of one of the states in theencoded optical signal, preferably the state with the highest amplitude.It is further shown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 10 GHz, and the offset is between 9.5-12.5GHz. Thus, in this example, the offset is selected to be between 0.95and 1.25 times the bandwidth of the opto-electrical converter. From thisexample, it can be seen that optimum LO frequency-offset varies littlewith the FM shift. In other words, the offset is varying little in therange between approximately 1 and 1.5 times the bandwidth of theopto-electrical converter. In this example, it has been shown that forlow dispersion, the optimum LO frequency-offset is independent of the AMextinction ration.

Example 20—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 1.5 Times the Bitrate (40 km SSMF, 5Gbps)

FIG. 20 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for a 40km SSMF system. The example as shown is modelled. Results for differentfixed AM extinction ratios are also included in the modelled results.Thus, in this example, it is shown that the frequency of the localoscillator is selected to have a predefined offset from, preferablyhigher than, the frequency of one of the states in the encoded opticalsignal, preferably the state with the highest amplitude. It is furthershown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 7.5 GHz, and the offset is between ca. 7-10GHz. Thus, in this example, the offset is selected to be between 0.9 and1.35 times the bandwidth of the opto-electrical converter. From thisexample, it can be seen that optimum LO frequency-offset varies littlewith the FM shift. In other words, the offset is varying little in therange between approximately 1 and 1.5 times the bandwidth of theopto-electrical converter. In this example, it has been shown that forrelatively low dispersion, the optimum LO frequency-offset isindependent of the AM extinction ration.

Example 21—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 2 Times the Bitrate (40 km SSMF, 5 Gbps)

FIG. 21 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for a 40 kmSSMF system. The example as shown is modelled. Results for differentfixed AM extinction ratios are also included in the modelled results.Thus, in this example, it is shown that the frequency of the localoscillator is selected to have a predefined offset from, preferablyhigher than, the frequency of one of the states in the encoded opticalsignal, preferably the state with the highest amplitude. It is furthershown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 10 GHz, and the offset is between ca. 9-13GHz. Thus, in this example, the offset is selected to be between 0.9 and1.3 times the bandwidth of the opto-electrical converter. From thisexample, it can be seen that optimum LO frequency-offset varies littlewith the FM shift. In other words, the offset is varying little in therange between approximately 1 and 1.5 times the bandwidth of theopto-electrical converter. In this example, it has been shown that forrelatively low dispersion, the optimum LO frequency-offset isindependent of the AM extinction ration.

Example 22—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 1.5 Times the Bitrate (80 km SSMF, 5Gbps)

FIG. 22 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for a 80km SSMF system. The example as shown is modelled. Results for differentfixed AM extinction ratios are also included in the modelled results.Thus, in this example, it is shown that the frequency of the localoscillator is selected to have a predefined offset from, preferablyhigher than, the frequency of one of the states in the encoded opticalsignal, preferably the state with the highest amplitude. It is furthershown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 7.5 GHz, and the offset is between ca. 5-9GHz. Thus, in this example, the offset is selected to be between 0.6 and1.2 times the bandwidth of the opto-electrical converter. From thisexample, it can be seen that optimum LO frequency-offset varies littlewith the FM shift. In other words, the offset is in this example varyinglittle in the range between approximately 0.5 and 1.5 times thebandwidth of the opto-electrical converter.

Example 23—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 2 Times the Bitrate (80 km SSMF, 5 Gbps)

FIG. 23 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for a 80 kmSSMF system. The example as shown is modelled. Results for differentfixed AM extinction ratios are also included in the modelled results.Thus, in this example, it is shown that the frequency of the localoscillator is selected to have a predefined offset from, preferablyhigher than, the frequency of one of the states in the encoded opticalsignal, preferably the state with the highest amplitude. It is furthershown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 10 GHz, and the offset is between ca. 7-12GHz. Thus, in this example, the offset is selected to be between 0.7 and1.2 times the bandwidth of the opto-electrical converter. From thisexample, it can be seen that optimum LO frequency-offset varies littlewith the FM shift. In other words, the offset is in this example varyinglittle in the range between approximately 0.5 and 1.5 times thebandwidth of the opto-electrical converter.

Example 24—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 1.5 Times the Bitrate (Back-to-Back, 10Gbps)

FIG. 24 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for aback-to-back system. The example as shown is modelled. Results fordifferent fixed AM extinction ratios are also included in the modelledresults. Thus, in this example, it is shown that the frequency of thelocal oscillator is selected to have a predefined offset from,preferably higher than, the frequency of one of the states in theencoded optical signal, preferably the state with the highest amplitude.It is further shown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 15 GHz, and the offset is between ca. 15-21GHz. Thus, in this example, the offset is selected to be between 1 and1.4 times the bandwidth of the opto-electrical converter. From thisexample, it can be seen that optimum LO frequency-offset varies littlewith the FM shift. In other words, the offset is in this example varyinglittle in the range between 1 and 1.5 times the bandwidth of theopto-electrical converter.

Example 25—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 2 Times the Bitrate (Back-to-Back, 10Gbps)

FIG. 25 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for aback-to-back system. The example as shown is modelled. Results fordifferent fixed AM extinction ratios are also included in the modelledresults. Thus, in this example, it is shown that the frequency of thelocal oscillator is selected to have a predefined offset from,preferably higher than, the frequency of one of the states in theencoded optical signal, preferably the state with the highest amplitude.It is further shown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 20 GHz, and the offset is between ca. 18-29GHz. Thus, in this example, the offset is selected to be between 0.9 and1.5 times the bandwidth of the opto-electrical converter. From thisexample, it can be seen that optimum LO frequency-offset varies littlewith the FM shift. In other words, the offset is in this example varyinglittle in the range between approximately 1 and 1.5 times the bandwidthof the opto-electrical converter.

Example 26—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 1.5 Times the Bitrate (40 km SSMF, 10Gbps)

FIG. 26 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 1.5 times the bitrate for a 40km SSMF system. The example as shown is modelled. Results for differentfixed AM extinction ratios are also included in the modelled results.Thus, in this example, it is shown that the frequency of the localoscillator is selected to have a predefined offset from, preferablyhigher than, the frequency of one of the states in the encoded opticalsignal, preferably the state with the highest amplitude. It is furthershown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 15 GHz, and the offset is between ca. 13-21GHz. Thus, in this example, the offset is selected to be between roughly0.9 and 1.4 times the bandwidth of the opto-electrical converter. Fromthis example, it can be seen that optimum LO frequency-offset varieslittle with the FM shift. In other words, the offset is in this examplevarying little in the range between approximately 1 and 1.5 times thebandwidth of the opto-electrical converter.

Example 27—Optimum LO Frequency-Offset from F1 as a Function of FM ShiftUsing a Photodiode Bandwidth of 2 Times the Bitrate (40 km SSMF, 10Gbps)

FIG. 27 shows how the optimum LO frequency-offset from F1 depends on theFM shift using a photodiode bandwidth of 2 times the bitrate for a 40 kmSSMF system. The example as shown is modelled. Results for differentfixed AM extinction ratios are also included in the modelled results.Thus, in this example, it is shown that the frequency of the localoscillator is selected to have a predefined offset from, preferablyhigher than, the frequency of one of the states in the encoded opticalsignal, preferably the state with the highest amplitude. It is furthershown that the offset is dependent on the bandwidth of theopto-electrical converter. In this example, the bandwidth of theopto-electrical converter is 20 GHz, and the offset is between ca. 22-32GHz. Thus, in this example, the offset is selected to be between roughly1.1 and 1.6 times the bandwidth of the opto-electrical converter. Fromthis example, it can be seen that optimum LO frequency-offset varieslittle with the FM shift. In other words, the offset is in this examplevarying little in the range between approximately 1 and 1.6 times thebandwidth of the opto-electrical converter.

Example 28—Optimum FM Shift as a Function of Photo-Detector Bandwidthand Transmission Distance (5 Gbps)

FIG. 28 shows an example of how the optimum FM shift depends on thephoto-detector bandwidth and transmission distance for 5 Gbps systems.From this example it can be seen that the frequency modulation isconfigured such that the frequency separation between the states in theoptical signal is dependent on the frequency bandwidth of theopto-electrical converter, in particular the frequency modulation isconfigured such that the frequency separation between the states in theoptical signal is proportional with a proportionality factor to thefrequency bandwidth of the opto-electrical converter. For theback-to-back system, the proportionality factor is approximately 1.2,whereas for a 40 km SSMF system, the proportionality factor isapproximately 1 and for a 80 km SSMF system, the proportionality factoris approximately 0.8. Thus, it can be seen that dispersion decreases theoptimum FM-shift.

Example 29—Optimum FM Shift as a Function of Photo-Detector Bandwidthand Transmission Distance (10 Gbps)

FIG. 29 shows an example of how the optimum FM shift depends on thephoto-detector bandwidth and transmission distance for 10 Gbps systems.From this example it can be seen that the frequency modulation isconfigured such that the frequency separation between the states in theoptical signal is dependent on the frequency bandwidth of theopto-electrical converter, in particular the frequency modulation isconfigured such that the frequency separation between the states in theoptical signal is proportional with a proportionality factor to thefrequency bandwidth of the opto-electrical converter. For theback-to-back system, the proportionality factor is approximately 0.8,whereas for a 40 km SSMF system, the proportionality factor isapproximately 0.4. Thus, it can be seen that dispersion decreases theoptimum FM-shift. Further, a higher photo-detector bandwidth increasesthe optimum FM shift.

The invention claimed is:
 1. A method for decoding an encoded opticalsignal comprising at least two different states, such as “0”-states and“1”-states, the optical signal being frequency and amplitude modulatedsuch that the different states are separated in frequency and amplitude,the method comprising the steps of: combining said encoded opticalsignal with light from a local oscillator configured with a localoscillator frequency; converting the combined local oscillator andencoded optical signal into one or more electrical signals by means ofat least one opto-electrical converter having a predefined frequencybandwidth, thereby providing an amplified and encoded electrical signalhaving one or more encoded signal current(s), where one type of stateshave a higher oscillation frequency than other type of states;rectifying the encoded signal current(s), thereby obtaining an encodedpower spectrum, wherein said power spectrum has different states, suchas “0”-states and “1”-states, with different power levels such that theycan be discriminated, said local oscillator frequency is defined by apositive local oscillator frequency-offset from the frequency of one ofthe states in said encoded optical signal; wherein said local oscillatorfrequency-offset is selected to be between 1 and 1.5 times thepredefined frequency bandwidth of the opto-electrical converter.
 2. Themethod according to claim 1, wherein said power spectrum is filtered bya low pass filter, thereby reducing the residual power of one type ofstates relative to another type of state.
 3. The method according toclaim 1, wherein said power spectrum is applied with thresholddetection, such that different states, such as “0”-states and“1”-states, are automatically detected.
 4. The method according to claim1, wherein said one of the states is a state with the highest amplitude.5. The method according to claim 1, wherein said local oscillator isoperating without a phase locked loop.
 6. The method according to claim1, wherein said local oscillator frequency-offset is greater than thebandwidth of the opto-electrical converter.
 7. The method according toclaim 1, wherein said local oscillator frequency-offset is selected tobe approx. 1.2 times the bandwidth of the opto-electrical converter. 8.A method for transmitting an optical signal, comprising the steps of:encoding the optical signal by amplitude and frequency modulation, anddecoding the combined AM and FM signal according to claim 1, and whereinsaid encoding or decoding of a combined AM and FM signal is using two ormore levels.
 9. The method according to claim 8, wherein said signal isencoded by one or more simultaneous AM and FM devices, such as afrequency chirped laser, in particular a DML or a VCSEL.
 10. The methodaccording to claim 8, wherein said signal is encoded by one or moreseparate AM device(s) and one or more separate FM device(s).
 11. Themethod according to claim 8, wherein said optical signal is configuredwith an AM extinction ratio between 3 dB and 6 dB.
 12. The methodaccording to claim 8, wherein said frequency modulation is configuredsuch that the frequency separation between the states in the opticalsignal is less than 15 GHz, or less than 14 GHz, or less than 13 GHz, orless than 12 GHz, or less than 11 GHz, or less than 10 GHz.
 13. Themethod according to claim 8, wherein said frequency modulation isconfigured such that the frequency separation between the states in theoptical signal is dependent on the frequency bandwidth of theopto-electrical converter.
 14. The method according to claim 8, whereinsaid frequency modulation is configured such that the frequencyseparation between the states in the optical signal is proportional witha proportionality factor to the frequency bandwidth of theopto-electrical converter.
 15. The method according to claim 14, whereinsaid proportionality factor is between 0.2 and 1.4.
 16. A detectorsystem for decoding a combined AM and FM encoded optical signalcomprising at least two different types of states, such as “0”-statesand “1”-states, comprising: a local oscillator configured with a localoscillator frequency; a coupling device configured for coupling theencoded optical signal with light from the local oscillator; one or moreopto-electrical converter(s) having a predefined frequency bandwidth,configured for providing an amplified and encoded electrical signalhaving one or more encoded signal current(s) where one type of stateshave a higher oscillation frequency than another type of states; arectifier configured for rectification of said signal current(s) toprovide a power spectrum, wherein said power spectrum has differentstates, such as “0”-states and “1”-states, with different power levelssuch that they can be discriminated, said local oscillator frequency isdefined by a positive local oscillator frequency-offset from thefrequency of one of the states in said encoded optical signal, whereinsaid local oscillator frequency-offset is selected to be between 1 and1.5 times the predefined frequency bandwidth of the opto-electricalconverter.
 17. The detector system according to claim 16, furthercomprising a low pass filter configured for reducing the residual powerof one type of states relatively to another type of state, such that“0”-states and “1”-states, with different power levels can bediscriminated more easily.
 18. The detector system according to claim16, further comprising a threshold detection module configured forthreshold detection of said power spectrum, such that different states,such as “0”-states and “1”-states, are automatically detected.
 19. Thedetector system according to claim 17, wherein the local oscillator (3)is an uncooled laser.
 20. The detector system according to claim 17,wherein the local oscillator (3) is a temperature controlled laser. 21.The detector system according to claim 16, wherein said one of thestates is a state with the highest amplitude.
 22. The detector systemaccording to claim 16, wherein said local oscillator frequency-offset isgreater than the bandwidth of the opto-electrical converter(s).
 23. Thedetector system according to claim 16, wherein said local oscillatorfrequency-offset is selected to be approx. 1.2 times the bandwidth ofthe opto-electrical converter(s).
 24. An optical communication systemcomprising at least one transmitter and at least one receiver comprisingthe detector system according to claim
 16. 25. The optical communicationsystem according to claim 24, wherein said transmitter is configured togenerate a combined AM and FM signal by one or more combined AM and FMdevice(s).
 26. The optical communication system according to claim 24,wherein said transmitter is configured to generate a combined AM and FMsignal by one or more separate AM devices and by one or more separate FMdevices.
 27. The optical communication system according to claim 24,wherein the transmitter is configured for performing the method ofcombining said encoded optical signal with light from a local oscillatorconfigured with a local oscillator frequency; converting the combinedlocal oscillator and encoded optical signal into one or more electricalsignals by means of at least one opto-electrical converter having apredefined frequency bandwidth, thereby providing an amplified andencoded electrical signal having one or more encoded signal current(s),where one type of states have a higher oscillation frequency than othertype of states; rectifying the encoded signal current(s), therebyobtaining an encoded power spectrum, wherein said power spectrum hasdifferent states, such as “0”-states and “1”-states, with differentpower levels such that they can be discriminated, said local oscillatorfrequency is defined by a positive local oscillator frequency-offsetfrom the frequency of one of the states in said encoded optical signal;wherein said local oscillator frequency-offset is selected to be between1 and 1.5 times the predefined frequency bandwidth of theopto-electrical converter, and wherein said encoding or decoding of acombined AM and FM signal is using two or more levels and wherein thereceiver is configured for performing the method of combining saidencoded optical signal with light from a local oscillator configuredwith a local oscillator frequency; converting the combined localoscillator and encoded optical signal into one or more electricalsignals by means of at least one opto-electrical converter having apredefined frequency bandwidth, thereby providing an amplified andencoded electrical signal having one or more encoded signal current(s),where one type of states have a higher oscillation frequency than othertype of states; rectifying the encoded signal current(s), therebyobtaining an encoded power spectrum, wherein said power spectrum hasdifferent states, such as “0”-states and “1”-states, with differentpower levels such that they can be discriminated, said local oscillatorfrequency is defined by a positive local oscillator frequency-offsetfrom the frequency of one of the states in said encoded optical signal;wherein said local oscillator frequency-offset is selected to be between1 and 1.5 times the predefined frequency bandwidth of theopto-electrical converter.